Providing a metal layer on a selected area of a surface of a polymer substrate in order to form a passage for transmitting electro-magnetic signals is widely applied in the fields of automobile, computer, communications, etc. The metal layer can be formed on the surface of the polymer substrate in various ways.
For example, U.S. Pat. No. 5,599,592 discloses a metallization process for metallizing a plastic composite article containing a polymer and grains of one or more metal oxides. The method includes steps of: 1) irradiating the plastic article surface to be metallized with a light beam emitted by an excimer laser; 2) immersing the irradiated article into at least one autocatalytic bath; 3) thermally processing the metallized plastic article to induce the diffusion of the deposited metal into the plastic article. The metal oxides may be oxides of antimony, aluminum, iron, zinc, or tin. It is disclosed in U.S. Pat. No. 5,599,592 that the content of the grains of metal oxides in the plastic composite article can be in the range from 1% to 30% (by weight). However, as described in the specification of U.S. Pat. No. 5,599,592, the contents of the grains of the metal oxides in the plastic composite articles are all above 4% by volume.